Take a swim in it. Does it still float you high enough that you are satisfied? Or if you want to be more precise, take 16 lbs of dive weights, attach them to the vest, and see if it sinks. Actually, you'd probably have to do some math to figure out how much more weight than your vest's rated flotation you'd need, based on the volume of your test weights...
I have a Lotus Sherman that is probably from 1997. It spent quite a few years in the closet not being used. It floats just fine, minimal fading, and the foam still feels nice and squishy. I have no qualms about using it whatsoever.
The Extrasport vest that I had before that was utterly worn out, faded, and sketchy looking after about 5 years. BUT, that was the one I used during my raft guiding/video boating days when I was on the water every day for months on end.
Sun exposure and dampness will break them down over time. If you use them a couple times a month and put them away dry and out of the sun, they will last a very long time. With a lot of use, I'd guess 2-5 years. A float test is a good indicator, but I have seen worn, faded PFDs that were blowing apart at the seams even though they would still float.
I have a Lotus Sherman that I got in about 1995. I have worn it, in preference to other PFDs I own, ever since.
The fabric is not sunbleached. The seams are all perfect. The straps and buckles are all OK. I haven't tested the flotation, but when I swim in it, it floats me just fine.
The USCG recommends replacing PFDs every five years. I recommend replacing them when they show detectable wear or cease to float your carcass.